Apparent Predation Risk Research Articles

Conservation implications of using an imitation carnivore to assess rarely used refuges as critical habitat features in an alpine ungulate.

Understanding relationships between animals and their habitat is a central goal in ecology with important implications for conservation. Misidentified habitat requirements can have serious repercussions because land protection or reintroductions might occur in less than optimal habitat. Studies of resource selection have greatly facilitated an understanding of ecological relationships but can be improved when vital yet infrequently utilized habitat features are more fully described. A critical element for many prey species is escape terrain or some other form of refuge to avoid predation. Mountain goats (Oreamnos americanus) are well known for their use of cliffs to avoid predation, but a survey of the literature revealed at least twelve different approximations of goat escape terrain, ranging from > 25° to > 50° slopes. Here, we seek to (1) enhance estimates of mountain goat escape terrain and antipredator behavior, and (2) highlight the limitations of the assumption that the time an animal spends in an area is proportional to importance. To improve estimates of goat escape terrain, we conducted field work across two years (2014–15) in Glacier National Park, Montana USA and manipulated apparent predation risk by exposing mountain goats to a threatening simulated grizzly bear (Ursus arctos) treatment and a non-threatening ungulate (control) treatment. Mountain goats moved in response to the simulated bear but not in response to the simulated ungulate, with shorter latencies to move for subjects in larger groups and at shorter distances to the simulated threat. Through a used-unused resource selection function we tested 22 landscape variables to describe the use of escape terrain. Proximity to slopes greater than 60° best explained the locations to which mountain goats fled after exposure to the simulated bear, and the average slope of these escape locations was 56.5° (±14.1 S.D.). Our results suggest that mountain goat escape terrain be considered at slopes of 60° as a minimum because our simulated threat did not include pursuit of goats and, thus, slopes of 60° are likely underestimates of actual escape terrain. Additionally, because direct interactions between carnivores and goats seldom occur, serious escape terrain is infrequently used. Past estimates of escape may have miscalculated the slopes which goats select for in response to predation risk. Based on experimental approaches in the wild, we suggest that anti-predator behavior should be included in studies of resource selection when the goal is to consider habitat as a predictor for conservation success. Finally, we discuss evidence suggesting a past mountain goat introduction failed due to lack of adequate escape terrain and subsequent recolonization of a predator.

Predation risk and space use of a declining Dall sheep (Ovis dalli dalli) population.

The abundance of ungulate populations may fluctuate in response to several limiting factors, including climate, diseases, and predation. In the northern Richardson Mountains, Canada, Dall sheep (Ovis dalli dalli) have undergone a major decline in the past decades and predation by grizzly bears (Ursus arctos) and wolves (Canis lupus) was suspected as a leading cause. To better understand the relationship between these three species located in this rugged and remote ecosystem, we relied on a combination of indirect methods. We investigated the apparent role of predation on the Dall sheep population using spatial ecology and stable isotopes. We examined seasonal variation in predation risk, focusing on how it may affect Dall sheep habitat use and sexual segregation, and we evaluated the proportion of Dall sheep in the diet of both predators using stable isotopes. The movements of the three species were monitored by satellite telemetry. Dall sheep habitat use patterns were analyzed using topographical features, greenness index, land cover, and apparent predation risk. The diets of grizzly bears and wolves were examined using a Bayesian mixing model for carbon and nitrogen stable isotopes. We found that Dall sheep habitat use varied seasonally, with different patterns for ewes and rams. Exposure to grizzly bear risk was higher for rams during summer, while ewes were further exposed to wolf apparent predation risk during winter. The importance of safe habitats for ewes was reflected in space use patterns. Stable isotopes analyses suggested that the diet of grizzly bears was largely from animal sources, with mountain mammals comprising about one quarter. Wolves mostly fed on both aquatic browsers and mountain mammals. Diet variation between individual predators suggested that some individuals specialized on mountain mammals, likely including Dall sheep. We conclude that grizzly bear and wolf apparent predation risk are important in driving Dall sheep habitat use and play a role in sexual segregation. Overall, this study presents an innovative combination of indirect methods that could be applied elsewhere to better understand predator-prey dynamics in remote ecosystems.

Effects of forest structure and harvest‐induced edges on predation risk

ABSTRACTManaged forest systems have implications for wildlife conservation because of their direct influence on condition and distribution of forest patches on the landscape. Spatial arrangement, structure, and composition of patches can influence reproductive outcomes for breeding birds by mediating nest site selection, survival, and predator–prey dynamics. Few studies have explored effects of local and adjacent forest‐stand structure and harvest‐induced edges on nest predation risk. Therefore, during summer 2014, we conducted artificial nest experiments within an intensively managed loblolly pine (Pinus taeda) landscape in Mississippi, USA. We used survival models to quantify effects of forest stand structure and proximity to harvest‐induced edges on apparent predation risk (APR). We conducted vegetation surveys and placed artificial nests (n = 146) at systematic distances from edges between 4 different pine stand types (Class 1: newly established stands; Class 2: 3–5 yr old; Class 3: 5–10 yr old; and Class 4: >10 yr old and postthinned). We sampled 4 stand pairs, each consisting of a Class 1 stand adjacent to a stand type of Class 1 (n = 4), 2 (n = 3), 3 (n = 4), or 4 (n = 4), and developed 15 a priori models relating APR to edge and vegetation characteristics. Apparent predation risk tended to be low and negatively related to edge where adjacent stands featured high proportion of shrub cover and vegetation density. We did not find strong evidence to support increased risk of nest predation near edges. These results were consistent with findings from studies in similar landscape contexts and suggest edges in forest‐dominated landscapes do not increase relative risk of nest predation. Rather, we found that APR was lowest in stands featuring dense shrub and vegetation cover that developed following clearcut harvest. Results of our study highlight some implications of forest harvest on reproductive success of birds, particularly those using early successional vegetation. © 2018 The Wildlife Society.

Safety from predators or competitors? Interference competition leads to apparent predation risk

Prey often react to predation risk by foraging preferentially in the safety of cover rather than in more risky open patches. Yet this pattern of patch use also can be caused by dominant interspecific competitors. We develop a simple theory of this form of apparent predation risk that describes the patch use of an optimal forager confronted with dominant individuals. The theory predicts that subordinate animals should increase their use of safe foraging patches as the density of nearby dominants increases. We tested the theory with meadow voles (Microtus pennsylvanicus) and southern red-backed voles (Myodes gapperi). We used dyadic encounters to confirm that meadow voles are dominant over red-backed voles. We then evaluated their respective foraging patterns in pairs of covered and open patches in 4 adjacent subgrids in an old-field enclosure. Subordinate redbacked voles foraged indifferently between covered and open patches when few meadow voles were present. Red-backed voles increased their use of both patches as the number of nearby meadow voles increased. Givingup densities were lowest, and harvesting efficiency highest, in covered patches when the number of nearby meadow voles was high. These results document competition between the 2 species and suggest that vigilance toward dominant meadow voles magnifies the risk experienced by red-backed voles in open patches. Investigators assessing foraging behavior between ‘‘safe’’ and ‘‘risky’’ patches might misinterpret the competitive effect as predation risk unless they 1st account for competition among foraging individuals.

Cautious cannibals: Behavioral responses of juvenile and adult blue crabs to the odor of injured conspecifics

Blue crabs are cannibalistic, and therefore the scent of injured conspecifics represents both a potential food cue, as well as an indicator of predation risk. We examined the response of blue crabs to conspecific odor alone, as well as in mixtures of attractive cues to determine how animals evaluate and respond to this odor. We explicitly manipulated risk-sensitivity based on either animal size (an indicator of susceptibility to predation) or hunger state (susceptibility to starvation) as ways to evaluate theories of risk-allocation, which suggest that decreases in predation risk, or increases in the risk of starvation, ought to result in diminished responses to sensory cues that signal predator presence or activity. Large and small blue crabs were challenged to locate the source of odor plumes consisting of the scent of injured conspecifics (risk cue), attractive food odors (attractive cue), or their mixture (conflicting cue). Neither large nor small blue crabs tracked aversive cues, but large blue crabs consistently tracked conflicting treatments to their source. Responses to conflicting and aversive treatments also involved diminished movement and reduced tracking speed relative to behaviors displayed in attractive plumes. Thus, even cannibalistic crabs seem to respond more prevalently to the apparent predation risk then to food reward, and risk-sensitive behaviors have a likely cost in terms of reduced food intake. Starved animals were more likely than unstarved animals to track conflicting plumes. Both the ontogenic shift and the response of starved animals support the notion that the cost of risk-aversive behaviors results in this strategy being allocated in proportion to the degree of potential risk. Since risk-aversive responses to chemical cues can produce strong effects in communities, the size-dependent nature of these responses in blue crabs may introduce considerable complexity in interactions between blue crabs, their predators, and their prey.

Factors Influencing Movement Distances and Home Ranges of the Short-tailed Shrew (Blarina brevicauda)

We studied movement distances and home ranges of Blarina brevicauda (northern short-tailed shrew) for 25 years in bluegrass, alfalfa, and tallgrass habitats in east-central Illinois. Whereas habitat and season influenced movement distances, population density and apparent predation risk did not. Specifically, movement distances were larger in tallgrass than in either alfalfa or bluegrass, presumably because of lower food availability in tallgrass as compared to the other two habitats. Movement distances of both sexes were slightly larger during the breeding season than the non-breeding season, perhaps reflecting searching for mates (males) and increased energetic demands of reproduction (females). Home-range areas did not differ between alfalfa (236 m2) and bluegrass (252 m2); we could not obtain reliable estimates from tallgrass. These data add to the limited information available on movements and home-range areas of the northern short-tailed shrew.

Threat‐Sensitive Behavioral Responses to Concentrations of Water‐Borne Cues from Predation

Aquatic organisms often detect predators via water-borne chemical cues, and respond by showing reduced activity. Prey responses may be correlated with the concentration of predation cues, which would result in graded antipredator behavioral responses that adjust potentially costly behavioral changes to levels that are commensurate with the risk of predation. Larvae of the predatory mosquito Toxorhynchites rutilus prey upon other container-dwelling insects, including larvae of the mosquito Ochlerotatus triseriatus. Previous work has established that O. triseriatus reduce movement, foraging, and time below the surface, and increase the frequency of resting at the surface, in the presence of water-borne cues from predation by T. rutilus. We tested whether these responses by O. triseriatus are threat sensitive by recording behavior of fourth instar larvae in two runs of an experiment in which we created a series of concentrations (100, 10, 1, 0.1, and 0.01% and 100, 70, 40, 20, and 10%) of water that had held either O. triseriatus larvae alone (control) or a T. rutilus larva feeding on O. triseriatus (predation). We also tested whether associated effects on time spent feeding are threat sensitive by determining whether frequencies of filtering or browsing are also related to concentration of cues. The frequencies of resting and surface declined, whereas frequency of filtering (but not browsing) increased more rapidly with a decrease in concentration of predation cues compared with control cues. Thus, O. triseriatus shows a threat sensitive behavioral response to water-borne cues from this predator, adjusting its degree of behavioral response to the apparent risk of predation.

The impact of long-term continuous risk of predation on two species of gerbils

Individuals of Gerbillus allenbyi Thomas, 1918 were subjected to artificial illumination in large field enclosures (2-ha sandy-substrate plots in the Negev Desert, Israel). The illumination was similar to that provided by a full moon and was used to mimic the elevated risk of avian predation that accompanies a full moon. We artificially illuminated the enclosures during all hours of darkness for 3 consecutive months. In some cases, we also added individuals of Gerbillus pyramidum Geoffroy, 1825 to provide a competitive challenge for the G. allenbyi. In the presence of the light source, individuals of G. allenbyi shifted their foraging activity to favor experimental areas of darkness. They also foraged less in the open and more beneath bushes. In the absence of the light source, G. allenbyi shifted its activity from the subplot with G. pyramidum to the subplot without the competitor. However, the competitive effect of the G. pyramidum disappeared in plots that were artificially illuminated. These results closely resemble those of earlier experiments during which we increased the apparent risk of predation in brief pulses lasting only 2 h/night.

Breeding suppression in free-ranging grey-sided voles under the influence of predator odour.

The breeding suppression hypothesis predicts that females of certain small mammal species will reduce reproduction as a response to the odour of a specialised mammalian predator. This was tested in a field experiment with grey-sided voles (Clethrionomys rufocanus) during three summer seasons (1997-1999) in the subalpine tundra of northern Norway, which is a natural habitat of this species. In a first phase free-ranging voles in six unfenced 1-ha plots were monitored by live-trapping from June to August each year. In a second phase from August to September, three of the plots were sprayed with weasel (Mustela nivalis) odour to simulate increased apparent predation risk, while the remaining three plots served as untreated controls. On all plots voles were individually marked with ear tattoos and were regularly live-trapped during the whole breeding season to follow their performance. On the treatment plots the recruitment rate of juveniles did not increase in late summer as it did on the control plots. The proportion of reproductively non-active adult females was significantly higher on the treatment plots for both old and young females. Our results thus verify the breeding suppression hypothesis for the first time under natural conditions. However, the response in overwintered females is in conflict with the original hypothesis because the assumed fitness benefits from breeding delayed until the next season are inaccessible to them. As an alternative explanation we propose a short-term response of reduced activity and interrupted breeding until the predator has exploited and left the feeding patch. Such a "duck and cover" strategy would increase the fitness of females of all age classes when prey habitats are patchy.

Female preference for large males in sailfin mollies, Poecilia latipinna : the importance of predation pressure and reproductive status

We examined the effect of predation risk on female association patterns in the live-bearing sailfin molly (Poecilia latipinna). We tested two classes of females, with and without the risk of predation by a green sunfish (Lepomis cyanellus): (1) postpartum females (maintained with males until visibly gravid, then isolated and tested within 24-48 h of dropping a brood); and (2) non-postpartum females (different females, isolated from males for >50 days). When there was no apparent risk of predation, postpartum females showed a significant preference for large over small males, whereas non- postpartum females showed no size preference at all. When there was an apparent risk of predation, postpartum females maintained their preference for larger males and non-postpartum females continued to show no preference for large or small males. These results suggest that reproductive status (receptivity) plays a greater role in mate preference than predation risk. For postpartum females, the cost of not choosing a preferred mate may outweigh the potential cost of predation. Non-postpartum females either do not benefit from mating or are being indecisive about mating and thus are less likely to be choosy whether or not a predator is present.

DOSAGE RESPONSE OF AN INDUCED DEFENSE: HOW SENSITIVE ARE TADPOLES TO PREDATION RISK?

Models of behavior and life history evolution under predation risk often assume that animals can detect and respond to subtle temporal and spatial variation in mortality risk, but there is little evidence supporting this assumption. We measured phenotypic responses of Rana lessonae tadpoles to variation in apparent predation risk signaled by different numbers of Aeshna dragonfly larvae consuming different quantities of tadpoles. The experiment took place in 80-L artificial ponds, and the predators were confined within cages so that they could not capture the experimental animals. There was good support for continuous dosage response curves for most behavioral and morphological traits, which indicates sensitivity to graded risk and therefore supports the assumptions of many models. Behavioral traits were most responsive to the number of tadpoles killed by the predators, whereas morphological traits responded to the number of dragonflies independent of the predators' diet. The results imply that behavioral and morphological responses can be triggered by different cues, and suggest that increasing investment in defensive traits entails increasing fitness costs.

Predation risk reduces male-male sexual competition in the Trinidadian guppy ( Poecilia reticulata )

Because agonistic behaviour can increase an individual's risk of predation, natural selection should favour individuals that modify their behaviour in the face of predation hazard in ways that maximise fitness. We investigated experimentally the effects of an increase in the apparent risk of predation on male-male competition within mixed-sex shoals of the Trinidadian guppy (Poecilia reticulata). We show that males engaged in significantly fewer aggressive interactions with conspecific males in the presence of a fish predator. However, they continued to court and attempt copulations with females at the same rate irrespective of the level of predation risk. In comparison, under predation hazard, female guppies were less responsive sexually and avoided fewer male copulation attempts. Such predator-mediated changes in male-male agonistic interactions and female sexual responsiveness towards males have important implications for the mode and intensity of sexual selection within populations.

Assessment and defence of solitary kangaroo rats under risk of predation by snakes

Prey that spend a large proportion of their time foraging must trade-off between predator defence and feeding. Because predation risk can be a cost in desert granivores, we predicted that kangaroo rats (Dipodomys) should weigh the risk of predation when deciding which behavioural option to pursue. We manipulated apparent predation risk of the giant kangaroo rat, D.ingens, and the desert kangaroo rat, D.deserti, and predicted they should be more vigilant and show greater defensive behaviour to the more dangerous stimulus of a real snake than to an artificial snake decoy. Results showed that kangaroo rats decreased foraging with increased vigilance when the snake and decoy were present. The largest decrease in foraging was in response to the live snake, and both species oriented towards, approached and footdrummed more in the presence of the live snake than the decoy. The species also differed in their antipredator behaviour: D.ingens footdrummed more and spent more time within striking distance of the snake than the decoy. Dipodomys deserti spent more time within 1m of the snake than the decoy and kicked sand at the snake. Sand kicking was never observed inD.ingens . We conclude that kangaroo rats are able to discriminate predation risk. They decrease foraging and increase vigilance in the presence of live snakes to assess risk and may approach and footdrum as a pursuit deterrent to communicate to the snake its chances of ambush are no longer available to cause the snake to leave. Sand kicking may function to harass the snake to cause it to leave.

Mate-choice copying under predation risk in the Trinigadian guppy (Poecilia reticulata)

Although females of numerous species possess genetically-based preferences for certain male traits and mate preferentially with males possessing these traits, recent theoretical and experimental evidence indicates that they may also copy (imitate) the mate choices of other females under certain circumstances. Such mate-choice copying is expected to be most prevalent when females have the opportunity to observe the mate choices of others and when mate choice is costly to females. One potential direct fitness cost of mate choice is increased individual risk of mortality due to predation. Here, we investigate for the first time the effect of increasing the apparent risk of predation on the tendency of females to copy die experimentally staged mate choice of another female. Using adult female guppies (Poecilia reticulata) originating from a Trinidadian population that experiences a relatively high fish predation pressure, we first establish that they possess a preference for the more brighdy colored of two simultaneously presented males in the absence of both mate-copying opportunity and an immediate threat of predation. However, most females reversed dieir initial mate preference when given an opportunity to copy the mate preference of another female in the absence of predation threat The proportion of females reversing their preference when given the opportunity to do so was not affected by increasing the apparent risk of (fish) predation. This result may be owing either to female guppies tending to copy the mate choice of others whenever the opportunity arises because the benefits of doing so accrue irrespective of the ambient risk of predation or to females choosing randomly between males with respect to their color pattern in the presence of the predator irrespective of mate-copying opportunity. These two explanations for die apparent lack of an effect of predation risk on mate-choice copying per se are both plausible but unfortunately could not be easily distinguished here. It may dius be possible, and interesting, that individual female guppies chose randomly between the available males in the presence of the predator but otherwise copied the choice of others when given the opportunity to do so.

Female mate choice under predation risk in the guppy

Sexual selection, and thus the evolution of both female mating preferences and preferred male traits, may be constrained by costs associated with mate choice. Recent theoretical models predict that female preference should decrease with increasing costs of mate choice such as predation risk. This general hypothesis was tested experimentally by presenting individual female guppies, Poecilia reticulata, originating from two Trinidadian populations with different predation regimes, with a choice of paired males (differing mainly in colour pattern) as potential mates in both the absence and presence of a cichlid fish predator that naturally co-occurs in one of these populations. Moreover, any reduction in female preference under risk was predicted to be positively correlated with the level of predation intensity occurring in their respective populations. In an experiment in which only the female could see the nearby predator, females from the high-predation population (Quaré River) significantly reduced both their overall level of sexual activity and their preference for a particular male (most commonly the more colourful male) in the presence of the predator. In contrast, the sexual activity level and mating preference of females from the low-predation population (Paria River) were unaffected by the apparent threat of predation. Observed changes in female sexual activity level and mating preference could not have been mediated by any change in male behaviour when the predator was present, because the males could neither see or smell the predator nearby. In a corollary experiment in which both males and females from the risk-sensitive Quaré River population could freely interact with one another and see the predator, females significantly reduced their preference for the more brightly coloured male in the presence of the predator, irrespective of predator-mediated changes in male courtship behaviour. These results are best explained by a reduction in female preference under increased apparent risk of predation, and are consistent with the general hypothesis that female mate choice in the guppy is potentially costly in terms of an increased risk of mortality due to predation and that females from different populations are differentially sensitive to this risk when assessing and choosing potential mates. Therefore, predators may exert direct selection on female mate choice in certain natural populations of the guppy.

Predation risk and alternative mating tactics in male Trinidadian guppies (Poecilia reticulata).

In the guppy (Poecilia reticulata), males have two alternative mating tactics. Individual males may either display to a receptive female prior to attempting to copulate with her or attempt to quickly sneakcopulate with a female without first displaying to her or without a prior receptive response from her. In this study, I experimentally investigated the effects of simulated local increases in the risk of predation (in the form of a cichlid fish predator model in situ) on the mating tactics used by free-ranging male guppies in two typical macrohabitats (riffle and pool) of a Trinidadian river. Focal male guppies displayed to females significantly less often on average, and conversely attempted sneak copulations more often, in the presence of the predator model than in its absence; this pattern was similar for both habitats. These fish therefore performed a lower proportion of sigmoid displays and increased their sneaky mating attempts when the apparent risk of predation had increased locally. This predator-mediated response is consistent with a trade-off between mating success and risk of mortality due to predation. The results are the first to confirm risk-sensitive mating behaviour in free-ranging male guppies within a population, and demonstrate the potential importance of predators in influencing the relative use of alternative mating tactics in this species on a microgeographical scale in the wild.

Substrate selection by juvenile Atlantic cod (Gadus morhua): effects of predation risk.

Although predator avoidance has been proposed as one possible factor influencing the distribution of fish among substrate types, no study has addressed this question directly. Groups of juvenile Atlantic cod were offered a choice between pairs of the following three substrates: sand, gravel-pebble and cobble. Their distribution on these substrates was compared prior to, during and following exposure to a predator (i.e. a larger conspecific). With no apparent risk of predation, juvenile cod preferred sand or gravel-pebble. When cobble was present, juveniles hid in the interstitial spaces of this substrate in the presence of a predator. With no cobble present, juveniles showed no preference between sand and gravel-pebble, and did not seek refuge from predation in association with these substrates. Following exposure to a predator (i.e. 2.5 h later) larger juvenile cod again showed a preference for the finer-grained substrates, but smaller individuals continued to associated with the cobble. The presence of cobble resulted in fewer juveniles being captured and a significant increase in the latency until the first juvenile was captured by the predator. Results are discussed with respect to the effects of predation on the distribution and survival of fishes among substrate types.

The influence of apparent predation risk on the foraging behaviour of eastern chipmunks (Tamias striatus)

The hypothesis that foraging behaviour is affected by the presence of a predator was tested in the field by presenting a stuffed marten (Martes americanus) to foraging eastern chipmunks (Tamias striatus). With a stuffed predator present, chipmunks approached the seed source more slowly. The closer the marten was to the seed tray, the smaller the loads collected by the chipmunks and the fewer the seeds consumed at the tray.

The Influence of Predation Risk on Central Place Foraging Variables in the American Robin, Turdus migratorius

A central tenet of optimal foraging theory is that organisms should maximize net energy intake. There has been a recognition lately, however, that predation risk may influence foraging decisions. Two foraging parameters are examined: bout duration and mean load of prey per bout, in American robins (Turdus migratorius) in relation to predation risk by American kestrels. Foraging adult robins nesting near an American Kestrel nest had increased mean bout duration and mean load of prey per bout compared to pairs without apparent predation risk. Central place foraging theory attempts to predict the optimal solutions to different foraging situations in cases where animals return to a central location to consume, cache, or deliver prey items (Andersson, 1981; Orians and Pearson, 1979; Schoener, 1979). For organisms that return more than one prey item to the central location (multiple-prey loaders), optimization criteria can be applied to predict load size and bout duration as functions of varying distance from the fixed point. The interpretation of data collected to test optimality models (Tinbergen, 1981) typically has relied on two assumptions: (1) the currency maximized is the rate of energy delivery to the nest; and (2) the primary constraint on energy maximization is the foraging capacity of the parents (the loading effect). Fagerstrom et al. (1983) outline constraints on energy maximization for central place foragers, and develop alternatives to the strategy of energy maximization. Organisms might lower nest visitation rates to minimally acceptable levels when predation risk is perceived (visit-minimization strategy). Under the threat of predation the rate of energy delivery should correspond to the minimum requirements of dependent offspring, resulting in 'Present address: Department of Plant Pathology and Ecology, The Connecticut Agricultural Station, P.O. Box 1106, New Haven, Connecticut 06504, USA. This content downloaded from 157.55.39.238 on Sat, 02 Jul 2016 06:02:54 UTC All use subject to http://about.jstor.org/terms 156 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE lower nest visitation rates and larger load sizes per foraging trip (bout). Since nest predation is a major source of mortality in many birds (Lack, 1954; Perrins, 1977), this visit-minimization strategy may be an important alternative to energy-maximization foraging in many birds. Variation in bout duration and load size in a multiple prey loader, the American Robin, is interpreted in terms of the visit-minimization strategy. Of three adult pairs studied, the territory of one pair was only 50 meters from a woodlot containing an American Kestrel (Falco sparverius) nest. Since kestrels are known to prey upon young robins (Bent, 1938, p. 113), it was predicted that this pair would exhibit longer bout durations and larger load sizes per bout relative to the other pairs where perceived predation risk was